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November, iqoj. 

No. 443. 




II. Arboreal Adaptations. 


In the struggle for existence it is apparent that single forms 
and whole groups of forms would independently become modified 
to a life off the ground. Very often only by such an adaptation 
could small defenseless animals save themselves from the attacks 
of larger and speedier carnivores. In addition, there is the ques- 
tion of food. The larger animals to whom the bulk of terres- 
trial food naturally goes are virtually absent from the trees. 
We accordingly find a multitude of animals that have made this 
region their abode where, freed from their enemies and with an 
abundance of food they have prospered. 


Order Marsupialia. 

Family Didelphyiclse (all except Chironectes). 
" Phalangeridae 

" Macropodkke (Dendrolagus only). 
" Dasyurkte (Dasyurus and Phascologale only). 



Order Edentata. 

Family Bradypodidae, Myrmecophagidas (only Tamandua and 

Order Ungulata. 
Sub-order Hyracoidea (Dendrohyrax). 

Order Carnivora. 

Family Felidae (many partly, the Jaguar alone truly arboreal). 

" Viverridae (the Fossa, Viverra and Arctictis). 

" Procyonidce (Procyon, Kinkajou, Bassariscus, Nasua 
and Bassaricyon) . 

Family Mustelidae (the Martens and Helectis). 
Family Ursidae (the Brown Bear). 

Order Rodentia. 

Family Anomaluridas. 
" Sciuridae (Sciurius). 
" Lophiomyidae. 
" Myoxidae. 

" Hystricidae (only the American sub-family Synethe- 
rinas) . 

Order Insectivora. 

Family Tupaiidaa. 

" Erinaceidae (Gymnura only). 
" Galeopithecidae. 

Order Cheiroptera. 

Order Primates. 
(All except Homo and the Baboons.) 

It will be observed from this list that with the exception of 
the Monotremata, the Cetacea and the Sirenia all the mamma- 
lian orders have arboreal representatives. Thus of the six 

No. 443] HABITS IN MAMMALS. 733 

existing marsupial families two are completely arboreal while 
arboreal forms are found in one or more representatives of each 
of the remaining families. Among those forms that are not 
arboreal there still persists a considerable number of vestigial 
structures and conditions in the pes all pointing unmistakably to 
a previous arboreal life. In like manner among the edentate 
sloths, many of the smaller Carnivora, Rodentia and Insec- 
tivora and finally the Cheiroptera and in large part the pri- 
mates have become arboreal. 

This adaptation however is probably a secondary one, acquired 
independently by the different orders. We should therefore 
expect diverse forms of the adaptation to exist. Here we shall 
distinguish the following main types : 

I. Partially arboreal. These are still capable of terrestrial 
life. Here belong the majority of the carnivores, insectivores, 
and rodents, and Dendrohyrax. 

II. Strictly arboreal. This contains the remaining" forms 
and is divisible into three sub-groups. 

(a) Modified for running on branches. — Arboreal marsupials 
and lemurs. 

(b) Modified for suspension from branches. — Sloths and bats. 

(c) Modified for swinging by fore limbs ; hind limbs on the 
marsupial type. — Remaining arboreal primates. 

It is clear that this classification expresses corresponding dif- 
ferences in foot structure. In the first group the pes is little 
different from the typical terrestrial running foot. The pha- 
langes have, as in the raccoons, become much elongated and the 
soles are often naked. In some cases a distinct plant igrady has 
replaced the previous digitigrady. 

It is in the second group that the greatest modification has 
occurred. In the first subdivision (a) the foot has become an 
almost perfect grasping organ ; the hallux being opposable ; the 
second and third digits have reduced and united ; the fourth toe 
is greatly elongated. There is also a distinct regression of the 
claws ; for as the foot becomes more and more prehensile in 
structure the nail is no longer indispensable and is lost (Dollo). 

In the second sub-division (b) of the second group, the manus 
and pes have become much elongated and centrally strengthened 


and the nails have been modified into hooks by means of which 
the body is kept in suspension. The number of digits is reduced 
to two in Cholcepus and three in Bradypus. The carpal and 
tarsal elements are laterally compressed and there is some anas- 
tomosis. This forms a more compact centre of resistance, while 
the proximal bones develop a more or less complete ball and 
socket joint in connection with the distal ends of the radius and 
tibia, to permit a more perfect rotation. 

In the last sub-division (c) both the manus and pes have 
become grasping organs. The hallux or pollex, or both, are 
generally opposable. Many modifications occur in the pes very 
similar to those already described for the marsupials. 

But in spite of these differences in main type there are 
developed certain important characters which distinguish arbo- 
real forms as a group from related terrestrial and aquatic types. 
These like responses to the same conditions are to be observed 
in what are otherwise most diverse forms. These characters are 
the following : 

i. The tail is often prehensile and, as in some of the Cebidae, 
naked at the tip being a sort of fifth arm with which the animal 
can move from branch to branch. Frequently correlated with 
this adaptation is the loss of the thumb. 

2. Ectodermal spines are often developed. These may 
occur on the root of the tail as in the Anomaluridse, on the 
shoulder or on the feet as in Gymnura and some of the Anthro 
poidea. In all these cases the spines are climbing organs. 

3. The limbs are much elongated. This elongation may 
occur in different segments in different forms. In the swinging 
apes, it is the fore-arm rather than the hand which is elongated. 
In the tree-sloths all the limb segments except the compressed 
carpal ia and tarsal ia and proximal phalanges are lengthened, the 
very long remaining phalanges and the claws forming a hook for 
suspension. In other forms the tarsals are greatly lengthened 
as in Tarsius, Galago and other lemurs. These elongations are 
obviously connected with the climbing and leaping habits of these 

4. The hallux or pollex, or both, are generally opposable. 
This gives the hand or foot a stronger hold on the branches and 

No. 443.] HABITS IN MAMMALS. 735 

is perhaps the most important element in the arboreal limb. It 
disappears however when the animal moves in suspension as in 
the sloths. 

5. The clavicle and scapula are well developed. These give 
strength to the fore extremities and thus increase the climbing 
power. It is interesting to observe that, as occurs in the Hystric- 
icke, the clavicles will be developed in one arboreal form while a 
terrestrial member of the same family will have vestigial clavicles 
or none at all. Together these two bones strengthen the pec- 
toral arch " in the transverse direction ; that is, against lateral 
strains of pulling and pushing, which came almost entirely from 
the use of the anterior limbs (Cope)." 

6. The ilium is broadened in some forms, particularly in 
Anthropoidea and the tree sloths. This adaptation is for the 
support of the viscera. In the edentates the pubis is directed 

7. In arboreal forms the ribs and chest are powerfully 
developed as compared with the conditions in their non-arboreal 

8. The number of the dorsolumbar vertebrae is often 
increased. It is in the tree sloths among the Edentata that 
the greatest elongation occurs. In the two-toed Choloepus the 
number is twenty-seven, and twenty-five in the species didactylus 
and hoffmanni respectively, while the number typical for the other 
forms of the order is about nineteen. In the three-toed Brady- 
pus the number is the typical nineteen. Curiously enough it is 
the cervical region which is here elongated there being nine cer- 
vical vertebras instead of six or seven as in the remaining Eden- 
tata. While one form has specialized itself to firm suspension 
the other has more or less sacrificed this character for a perhaps 
more valuable one — the mobile neck. Among the Rodentia 
where the typical number of dorsolumbars is nineteen, Capromys 
which is arboreal possesses twenty-three. Hyrax and Dendro- 
hyrax have thirty and twenty-eight respectively ; fully six more 
than that prevailing among the terrestrial ungulates. 

If inverse evidence can be of any value, it is known that in 
the human species, ancestrally adapted to arboreal life, there is 
a tendency toward the shortening of the back ; there being gen- 


erally, one less vertebra in man than in the still arboreal apes. 
On the other hand among the marsupials where typically arbo- 
real forms prevail the number is constant for the group — nine- 
teen. This may be due to the fact as Dollo has shown that the 
terrestrial forms have but very lately modified themselves to this 
mode of life — the whole group of marsupials having been at one 
time arboreal. In like manner the number in the carnivores is 
constant (twenty). In this group the arboreal forms have but 
lately diverged from their terrestrial relatives. In the Insec- 
tivora there is also no difference of any significance. 

Among isolated adaptations may be mentioned the modified 
feet of Hyrax and Dendrohyrax. As described by Dobson 
these animals are enabled to climb perpendicular walls and trees 
without the use of claws ; nor is there an opposable hallux or 
pollex. The thickly padded tuberculated soles are drawn up 
by certain flexor muscles thus leaving a vacuum by means of 
which the animal retains its hold. In the Cercolabidse there are 
in addition to other arboreal characters such as spines, tubercles 
on the soles which may serve as in Hyrax. 


Ai.i.kx, H. 

"33. A Monograph of the Bats >f Njrth America. Bull. U. S. Natl. 
Mas. No. 43. 
Bkddard, F. E. 

.02.. Mammalia. Cambridge Nat. Hist. Vol. 10. 
Con:, E. D. 

'89. The Mechanical Causes of the Development of the Hard Parts 
in the Mammalia. Journ. Morph., Vol. 3. 
Doiisox, G. E. 

'76. On Peculiar Structures in the Feet of Certain Species of Mam- 
mals which enable them to walk on Smooth Perpendicular Surfaces. 
Proc. Zoo!. Soc. 1.876. London. 
Doli.o, L. 

'98. Les ancea'es des Marsupiaux etaient-ils arboricoles? Trav. Stat. 
Zool. Wimereux, Tom. 7. 
Flower, W. H. and Lydekkkii, R. 

'91. An Introduction to the Studv of the Mammals Living and 
Muybridge, E. 

'77. Animal Locomotion.